1. Field of the Invention
The present invention relates to an accumulator blow-back hydraulic circuit and, more particularly, to a hydraulic circuit for assuring proper blow-back of an aircraft flight control surface to a neutral position in the event of failure of the hydraulic circuit. The invention moves the control surface to its neutral position with great accuracy and little flutter while providing a controlled rate of travel. The blow-back circuit allows the use of a geared rotary actuator (GRA) system which provides high stiffness and low backlash at the control surface.
2. Description of the Prior Art
It is known in mechanical actuation systems for aircraft flight control surfaces to have some mechanism for blowing back or backdriving the surfaces under air loading to an aerodynamically neutral panel position. This feature permits an aircraft to continue flight operations until a safe landing is achievable.
However, I have found that the conventional blow-back mechanisms do not sufficiently address a number of concerns among aircraft manufacturers, namely the blow-back rate, the proximity of the control surface panel, and the flutter resistance of the panel after reaching the neutral position. In the past, actuating systems utilized a power-hinge geared rotary actuator to maintain high stiffness and low backlash of the panel for assuring proper control of the surfaces. The GRA system has the disadvantage of a large mechanical power loss, i.e. a relatively low backdriving efficiency, which results in high backdrive loads at the control surfaces. It also typically requires a large overall gear ratio to the hydraulic motor of the power drive unit which amplifies drag loads at the control surfaces. As a result, proper blow-back of control surfaces to a neutral position rapidly, accurately and permanently has been almost impossible to achieve.
In the past, accumulators have been employed to obtain emergency control, particularly in steering systems, upon interruption of the power fluid supply For example, U.S. Pat. No. 4,476,677 shows an oil/gas accumulator in combination with a charge-discharge valve. However, such a system would not be suited for driving control surfaces of an aircraft to a blow-back position while providing high stiffness and low control surface backlash where the surface is locked in its neutral position. Similar systems are shown in U.S. Pat. Nos. 4,422,290; 4,326,558; and 4,217,968.
Another proposal for using accumulators in association with a valve in emergency situations is shown in U.S. Pat. No. 4,065,094. To prevent catastrophic accidents in power generating plants, quick closing of a valve to prevent fluid leakage is provided by active and standby circuit accumulators.
Hydraulic circuits for motor vehicles in which an accumulator is used are known as shown in U.S. Pat. No. 3,898,809. This system uses a fluid actuated brake booster and a pump outlet connected to a steering valve. A valve spool has an end exposed to a working pressure of the brake booster and in response to that working pressure throttles fluid flow from the pump outlet side and maintains the pressure of the pump outlet side at a predetermined pressure differential above the working pressure.
The storage of hydraulic power using accumulators is shown in U.S. Pat. No. 3,680,311. This circuit uses an accumulator, a safety accumulator and a safety distributor in addition to feed pressurized fluid from the main accumulator to a first and second orifices of an operating element. The safety accumulator is connected to the second orifice of the operating element in parallel with a second outlet duct of the main distributor. This arrangement apparently finds particular application in immersed producing or drilling well heads where the installation has to be controlled remotely. This circuitry is not concerned with the problems associated with blow-back.
A stroke-completing mechanism using an accumulator is disclosed in U.S. Pat. Nos. 3,651,642 and 3,225,544. The former patent involves textile machinery and does not have elements in the circuit which could effect blow-back, high stiffness and low backlash. The latter patent involves the use of an accumulator with associated valves to close a gate in a steel mill bin in response to an air pressure interruption.
Insofar as aircraft systems are concerned, U.S. Pat. No. 2,835,460 shows a hydraulic circuit for an aircraft control surface in which a feel simulator is provided in the event of main pressure supply failure. A pressure accumulator and a control valve are associated with a pilot's control jack to give the pilot time to adjust to changed conditions resulting from the emergency, thereby preventing the pilot from exerting excessive pressure upon the control stick. In such a system, there was no concern with driving a fluid motor to a neutral position and thereafter cutting off the motor charge pressure to prevent further control surface movement.
U.S. Pat. No. 2,505,206 discloses a propeller feathering circuit which, similar to the patent discussed in the immediately preceding paragraph, uses an accumulator; in this case the accumulator is permitted by a valve to complete feathering in response to a differential pressure between the accumulator and pump pressures. Here again, such a conventional system does not suggest a circuit for solving problems involved with control surface blow-back.